1. Field of the Invention
This invention relates generally to processes and materials used in the fabrication of a giant magnetoresistive (GMR) sensor, and more specifically to the use of a novel specularly reflecting nano-oxide layer (NOL) as a capping layer of a bottom spin valve sensor structure to improve its GMR ratio.
2. Description of the Related Art
One of the most commonly used structural configurations of magnetic and non-magnetic layers in giant magnetoresistive (GMR) read-heads is the so-called spin-valve magnetoresistive (SVMR) structure. In the most basic version of the SVMR, two ferromagnetic layers such as CoFe or NiFe are separated by a thin layer of electrically conducting but non-magnetic material such as Cu. One of the layers has its magnetization direction fixed in space or “pinned,” by exchange coupling with an antiferromagnetic (AFM) layer, usually a layer of MnPt, directly deposited upon it. The remaining ferromagnetic layer, the unpinned or free layer, can rotate its magnetization vector in response to small variations in external magnetic fields such as are produced by moving magnetic media, (which variations do not affect the magnetization direction of the pinned layer). The rotation of one magnetization relative to the other produces changes in the resistance (magnetoresistance) of the three layer structure, the changes being dependent on the cosine of the angle between the two magnetization vectors. As a result of these resistance variations, a constant “sense” current sent through the SVMR produces voltage variations across it, which are sensed by external circuitry. This effect of magnetization directions on electrical resistance is a result of spin dependent electron scattering, wherein the orientation of the electronic spins of the electrons in the sense current relative to the magnetization of the layer directly affects their scattering cross-sections and, consequently, the resistance of the magnetic material. Newer versions of the spin valve configuration make use of a pinned layer which is a triply laminated layer comprising two ferromagnetic layers magnetically exchange coupled by a thin, non-magnetic “spacer” or “coupling” layer. The two ferromagnetic layers are coupled in a manner that has their respective magnetizations maintained in antiparallel directions by the antiferromagnetic pinning layer. This type of composite pinned layer is termed a “synthetic antiferromagnetic (SyAF) pinned layer,” SyAP for brevity.
An older version of the use of variable magnetoresistance as a sensing tool was the anisotropic magnetoresistive (AMR) effect, wherein the resistance of a magnetic material was found to depend upon the angle between its magnetization and the direction of a current through it. The discovery of ways to enhance the magnetoresistive effect by the use of two layers of magnetic material (the spin valve) rather than one and by the methods used to form these layers (eg. the SyAF pinned layer), has led to what is now called the giant magnetorsistive (GMR) effect. It is this GMR which will be the subject of the present invention.
The major figure of merit for SVMR performance is its magnetoresistive ratio dR/R (usually expressed as a percentage), which is a measure of the maximum variation of its resistance that can be expected in operation. Improvements in the magnetoresistive ratio of a sensor element can be expected if the electrons in the sense current spend more time within the magnetically active portions of the sensor. For example, if the sensor element contains electrically conductive layers which do not directly contribute to the magnetoresistive effect (eg. they are not magnetic), then portions of the sense current may be shunted through these layers and not contribute to voltage variations across the sensor. It is now generally well accepted that a major contribution to the GMR effect is the presence of interfaces between various layers of the sensor elements. These interfaces produce specular reflection of the electrons, effectively removing mean-free-path limitations on electron scattering that would normally be placed on them by the external dimensions of the sensor. The realization of the importance of internal reflections on the magnetoresistive ratio, has produced great interest in the formation of sensor elements that exploit these interfacial scattering effects. For example, various types of capping layers, seed layers, buffer layers and nano-oxide layers (NOL) have been proposed as mechanisms for improving magnetorsistive ratios of sensor elements.
Huai et al. (U.S. Pat. No. 6,222,707 B1) teaches a method in which a seed layer is used to provide an improved texture for an antiferromagnetic layer grown upon it. The seed layer allows the growth of improved forms of antiferromagnetic pinning layers in bottom spin valves (spin valves in which the pinned layer is vertically beneath the free layer) thereby improving the exchange coupling between the pinning and pinned layers and, consequently, improving the magnetoresistive ratio.
Huai et al. (U.S. Pat. No. 6,175,476 B1) provides a bottom spin valve sensor having two antiparallel pinned layers coupled by a high resistivity rhenium layer that reduces shunt current through the three-piece pinned layer while still retaining adequate coupling between the two antiparallel layers. The sensor also includes a Ta capping layer whose purpose is to prevent oxidation of the sensor.
Gill (U.S. Pat. No. 6,181,534 B1) teaches a method for forming a magnetoresistive spin valve sensor element in which copper and nickel oxide specular reflection layers are formed on each other and over a free magnetic layer.
Pinarbasi (U.S. Pat. No. 6,208,491 B1) teaches the formation of a capping structure comprising layers of CoFe and Ta or, alternatively CoFe, Cu and Ta, which improves the magnetoresistive performance subsequent to long periods of time at high temperatures.
Lee et al. (U.S. Pat. No. 5,731,936) teaches the formation of an MR sensor having a capping layer that can be either a Ta layer, an NiFeCr layer, an NiCr layer, an NiCr/Ta layer, or a Ta/NiCr layer. It is claimed that either the NiFeCr layer or the NiCr layer provide improved sensor thermal stability as compared to a single Ta capping layer.
Kim et al. (U.S. Pat. No. 5,637,235) provide a bottom spin valve sensor having a Ta capping layer of between 0-100 angstroms thickness to protect the upper surface of the top ferromagnetic layer.
The literature also contains reports of magnetoresistive ratio improvements as a result of the inclusion of novel materials and structures in the fabrication of sensors. In this regard, Swagten et al., in “Specular Reflections in Spin Valves Bounded by NiO Layers,” IEEE Transactions on Magnetics, Vol. 34, No. 4, July 1998, pp. 948-953, report on achieving increased electron reflectivity by an insulating NiO layer that is used to exchange bias a spin valve. Swagten et al., in “Enhanced giant magnetoresistance in spin-valves sandwiched between insulating NiO,” Phys. Rev. B, Vol. 53, No. 14, 1 April, 1966 also report on the enhanced GMR effects obtained when sandwiching Co/Cu/Co and Ni80Fe20/Cu/Ni80Fe20 between layers of NiO.
Y. Kamiguchi et al., in “CoFe Specular Spin Valve GMR Head Using NOL in Pinned Layer,” Paper DB-01, Digest of Intermagnetic Conference 1999, report on a spin valve structure in which the pinned layer contains a nano-oxide layer (NOL) which enhances specular electron scattering.
T. Mizuguchi and H. Kano, in “Characteristics of spin-valve films with non-magnetic oxide layers for specular scattering,” Paper EB-12, Digest of MMM/Intermag. 2001 conference, p. 263, report on a new spin valve structure in which the coercivity of the free layer remains low while the GMR properties are improved. In their structure, the free layer is separated by a Cu layer from a top TaO specularly reflecting capping layer and there is another specularly reflecting RuO layer incorporated within the pinned layer.
Y. Huai et al., in “Highly Sensitive Spin-Valve Heads with Specular Thin Oxide Capping Layers,” Paper EB-14, Digest of MMM/Intermag. 2001 Conference, p. 263, discuss the specular reflection enhancing effects of thin oxide capping layers used in bottom synthetic specular spin-valve structures.
The present invention provides a method of improving the GMR ratio of a bottom spin-valve structure by the use of a novel NOL material layer as its capping layer.